1. Field of the Invention
The present invention relates to an improved injection plating apparatus for plating the external leads of a semiconductor product.
2. Description of the Related Art
FIGS. 1 to 3 shows a conventional injection plating apparatus. FIG. 2 is a cross sectional view of the apparatus of FIG. 1 taken along the line I--I' and FIG. 3 is a cross sectional view of the apparatus of FIG. 1 taken along the line II-II'.
A plating liquid is injected from a plating liquid reservoir to a cavity 13 through pressure boxes 11 and 12. A semiconductor product 14 is compressed by cavity boxes 15 and 16 from both sides. Electrodes 17 are formed on portions of inner surfaces of the cavity boxes (portions opposed to each other and separated from external lead surfaces). When the plating liquid is injected, a positive voltage is applied to the electrodes and a negative voltage is applied to lead frames 18.
With injection plating apparatus of this type, a semiconductor product having external leads arranged at regular intervals on both sides can be satisfactorily plated. However, if the leads of a semiconductor product are not arranged at regular intervals as shown in FIGS. 4A, 4B and 5, the thickness of a plate layer formed thereon cannot be made uniform.
For example, in an SOJ 26-pin package, three central pins are missing on each side. Thus, the package actually has 20 pins. When the external leads of such a semiconductor product are plated with the above-mentioned injection plating apparatus, end pins (Nos. 5, 9, 18 and 22 shown in FIG. 4B) are plated thicker than the other pins for the following reasons: first, plating liquid flows faster in a central portion (lead-missing portion) than any other portion; and second, currents are concentrated on the end pins 5, 9, 18 and 22 shown in FIG. 4B.
FIG. 6 shows the thickness of plate layers formed on the respective pins (Nos. 1 to 13 shown in FIG. 5) which have been subjected to a plating process by the above-mentioned injection plating apparatus. As clearly shown in FIG. 6, the thickness of the plate layers on the end pins (Nos. 5 and 9 shown in FIG. 5) is about 80% to 90% greater than those of the other pins.